Fossil fuels will remain the major source of energy for many years to come. Diesel fuels form the major chunk of fossil fuels. Globally the demand pattern of fuel indicates that it is shifting towards diesel even in those countries where the traditionally gasoline was dominant fuel. Increasing environmental concerns and climatic awareness made the diesel specifications tighter and tighter with respect to sulfur and cetane for controlling vehicular emissions. Due to tighter sulfur specifications, the sulfur levels in diesel fuel is going down, very soon 10 ppm sulfur diesel fuel will be a worldwide norm.
Hydroprocessing is the most generally used process to achieve these specifications in refineries today. It involves subjecting a hydrocarbon feedstock along with hydrogen gas under catalytic processing at high temperature and pressure conditions suitable to achieve the product specifications. The process generally can be classified in two major classes, first one is hydrotreating, where there is no major change in molecular weight of feedstock occurs, only heteroatoms such as those related to emission norms e.g. sulfur content and those heteroatoms which may hinder removal of these atoms e.g. nitrogen are mainly removed. In addition to this removal of heteroatoms from organic molecules some minor changes in molecules are also obtained in order to achieve other specifications such as cetane number. These changes involves the mainly saturation of aromatic molecules to respective naphthenes. The second class of hydroprocessing is the hydrocracking, where there is conversion by means of cracking of heavy molecules to lighter (more usable) molecules in presence of high hydrogen pressures. The catalysts also differ in two processes owing to their duties to be performed. In hydrotreating the catalyst involved is having only metal function on inert support and in hydrocracking the metal function is supported on acidic support rather than inert support, which additionally gives cracking activity to the catalyst.
There are other types of processes which are emerging or being practiced to achieve the abovementioned goals of product fuel specifications, such as FCC for catalytic conversion of heavier molecules to lighter ones and oxidative desulfurization processes to achieve sulfur specifications. But all these processes can achieve only one of the specifications; for example, FCC can convert the heavier molecules to lighter ones but the products from which need again to be treated for heteroatom removal and cetane specifications in case of diesel. The oxidative desulfurization process may meet the sulfur specification but not cetane number. Therefore, hydroprocessing will be the only way to achieve all the product fuel specifications. The hydroprocessing thus have emerged as major important process in refining field, second only to crude distillation.
The desulfurization & cetane improvement of diesel boiling range hydrocarbon in hydrotreating process is achieved by reacting with hydrogen in presence of catalyst at high temperatures and high pressures. Extensive amount of work is being done to increase the effectiveness of the hydroprocessing to achieve desired product specifications with economical considerations. The developments are being done in the various areas of catalysis, process design and equipment designs. Various processing schemes are also being suggested to increase the effectiveness of hydroprocessing.
Gupta in U.S. Pat. No. 5,705,052 described a configuration of hydroprocessing which comprises achieving the two or more reaction stages in a single reaction vessel with hydrogen being circulated from last reaction stage to first reaction stage. The inter-stage gas and liquid separation along with liquid stripping is done in external vessel which again act a multistage liquid stripper but all the gases combined and recycled to last reaction stage.
Ackerson et. al. in U.S. Pat. Nos. 6,123,835 & 6,881,326 described a liquid phase hydroprocessing where need to circulate hydrogen through catalyst is eliminated. The hydrocarbon feedstock is presaturated and fed to the catalyst bed. The hydrogen required is supplied in dissolved form only. The solubility of feedstock is enhanced with the addition of dilution solvent which can be product of the process itself.
Turner in U.S. Pat. No. 7,238,274 and Stupin et. al. in U.S. Pat. No. 7,238,275 invented an integrated hydrotreating process for two feedstocks of different boiling range. The configuration involves mixing of vapor part of effluent of heavier feedstock hydrotreating reactor (1st) with portion of lighter feedstock and hydrotreating in second reactor and separating the vapor part and recycling the same after make up to first hydrotreating reactor.
Leonard et. al. in U.S. Pat. No. 7,842,180 suggested a innovative scheme for hydrocracking process where a effluent of hydrocracking reactor is mixed with fresh feed and hydrogen at low concentration and treated in hydrotreating reactor; the effluent of which is cooled and fractionated and unconverted oil along with low hydrogen flow goes to hydrocracking reactor.
Leonard et. al. in U.S. Pat. No. 7,794,585 gave a method of hydroprocessing hydrocarbon streams, involving configuration of firstly directing hydrocarbonaceous feedstock to a first substantially liquid phase hydroprocessing (hydrotreating) zone and the effluent from the first substantially liquid phase hydrotreating zone to a second substantially liquid phase hydroprocessing (hydrocracking) zone generally undiluted with other hydrocarbon streams and then recycling a liquid portion of hydrocracking which preferably includes an amount of dissolved hydrogen therein to the hydrotreating zone.
Kokayeff et. al. in U.S. Pat. No. 7,794,588 described a process for producing ULSD having reduced polyaromatics with the configuration of firstly desulfurization at low pressure to obtain ULSD with minimum saturation of aromatics and then saturation of polyaromatics at high pressure with very low hydrogen rates without liquid recycle and without dilution with solvent, etc.
Kokayeff et. al. in U.S. Pat. No. 7,799,208 described a hydrocracking process having first gas phase continuous hydrotreating and then separating the hydrotreater effluent in gas and one or more liquid portions. Combining one or more liquid portions or bottom portion from separator with low hydrogen and passing the mixture in continuous liquid phase form (with fine hydrogen bubbles) to hydrocracker reactor and combining the hydrocracker effluent with hydrotreater effluent without liquid recycle and without dilution with solvent, etc.
Kokayeff et. al. in U.S. Pat. No. 7,790,020 gave a process for producing ULSD having higher cetane number, the configuration involves: firstly desulfurization at low pressure (48 barg) to obtain ULSD with minimum saturation of aromatics and then saturation of aromatics at high pressure (69 barg) with very low hydrogen rates without liquid recycle and without dilution with solvent, etc.
Kalnes in U.S. Pat. No. 6,328,879 described a process for simultaneous hydroprocessing of two feedstocks where first hydrocarbon feed (heavy) is contacted with hydrogen in hydrocracker, the effluent from which is separated/stripped in gas and liquid. The portion of this liquid is recycled to hydrocracker and a second hydrocarbon feed (lighter than first) is introduced in separator/stripper as reflux. The gas from separator/stripper is passed to post-treat hydrotreater for aromatics saturation and recycling the portion of gas from the post-treat hydrotreater to hydrocracker
It may suffice to say that in spite of extensive amount of research work already available in the art, there are scopes for continuous improvement of the hydroprocessing. The prior art available suffers from the disadvantage of not addressing the issue of formation of recombinant mercaptans towards end regions of hydroprocessing whether it is hydrotreating or hydrocracking, while attempting to obtain ultra low sulfur diesel levels of less than 10 ppm, this issue becomes of prime importance. Further, though there are various schemes available for multistage reaction in hydroprocessing, but they either suffer from the disadvantage of cost intensiveness or complexity of designs and operability. The proposed invention is an attempt to overcome these shortcomings in the present art of hydroprocessing scheme.